This invention relates generally to cryogenic air separation and, more particularly, to cryogenic air separation for the production of oxygen.
Oxygen is produced commercially in large quantities by the cryogenic rectification of feed air. The capital cost of cryogenic air separation plants to produce product oxygen in large volumes is quite high and any arrangement which serves to reduce these costs would be highly desirable.
One way to reduce the cost of high volume oxygen plants is to operate the plants at elevated pressures which would reduce the size and thus the cost of major components of the plants such as the columns and the main condenser. Unfortunately the compression costs to achieve such elevated pressures generally negate the resulting savings in capital costs.
Accordingly, it is an object of this invention to provide a cryogenic air separation system for producing oxygen which can operate at elevated pressures and wherein compression costs do not overcome savings in capital costs which may be achieved by the operation at elevated pressure.
The above and other objects, which will become apparent to those skilled in the art upon a reading of this disclosure, are attained by the present invention, one aspect of which is:
A method for producing oxygen by the cryogenic separation of feed air comprising:
(A) compressing feed air and passing the compressed feed air into a cryogenic air separation plant comprising a higher pressure column and a lower pressure column;
(B) combusting air and fuel to produce hot gas, expanding the hot gas in a gas turbine to produce power, and using essentially all of the power produced by the gas turbine to carry out the compression of the feed air; and
(C) producing oxygen by cryogenic rectification within the cryogenic air separation plant, and recovering oxygen withdrawn from the cryogenic air separation plant as product.
Another aspect of the invention is:
Apparatus for producing oxygen by the cryogenic separation of feed air comprising:
(A) a cryogenic air separation plant comprising a higher pressure column and a lower pressure column, and a compression system comprising at least one compressor for compressing feed air for passage into the cryogenic air separation plant;
(B) a gas turbine for producing power by the expansion of hot gas and configured such that essentially all of the power produced by the gas turbine is used to operate the compression system; and
(C) means for recovering product oxygen taken from the lower portion of the lower pressure column of the cryogenic air separation plant.
As used herein the term xe2x80x9ccolumnxe2x80x9d means a distillation or fractionation column or zone, i.e. a contacting column or zone, wherein liquid and vapor phases are counter currently contacted to effect separation of a fluid mixture, as for example, by contacting of the vapor and liquid phases on a series of vertically spaced trays or plates mounted within the column and/or on packing elements such as structured or random packing. For a further discussion of distillation columns, see the Chemical Engineer""s Handbook, fifth edition, edited by R. H. Perry and C. H. Chilton, McGraw-Hill Book Company, New York, Section 13, The Continuous Distillation Process. 
The term xe2x80x9cdouble columnxe2x80x9d is used to mean a higher pressure column having its upper portion in heat exchange relation with the lower portion of a lower pressure column. A further discussion of double columns appears in Ruheman xe2x80x9cThe Separation of Gasesxe2x80x9d, Oxford University Press, 1949, Chapter VII, Commercial Air Separation.
Vapor and liquid contacting separation processes depend on the difference in vapor pressures for the components. The high vapor pressure (or more volatile or low boiling) component will tend to concentrate in the vapor phase whereas the low vapor pressure (or less volatile or high boiling) component will tend to concentrate in the liquid phase. Distillation is the separation process whereby heating of a liquid mixture can be used to concentrate the more volatile component(s) in the vapor phase and thereby the less volatile component(s) in the liquid phase. Partial condensation is the separation process whereby cooling of a vapor mixture can be used to concentrate the more volatile component(s) in the vapor phase and thereby the less volatile component(s) in the liquid phase. Rectification, or continuous distillation, is the separation process that combines successive partial vaporizations and condensations as obtained by a countercurrent treatment of the vapor and liquid phases. The countercurrent contacting of the vapor and liquid phases can be adiabatic or nonadiabatic and can include integral (stagewise) or differential (continuous) contact between the phases. Separation process arrangements that utilize the principles of rectification to separate mixtures are often interchangeably termed rectification columns, distillation columns, or fractionation columns. Cryogenic rectification is a rectification process carried out at least in part at temperatures at or below 150 degrees Kelvin (K).
As used herein the term xe2x80x9cindirect heat exchangexe2x80x9d means the bringing of two fluids into heat exchange relation without any physical contact or intermixing of the fluids with each other.
As used herein the term xe2x80x9cproduct oxygenxe2x80x9d means a fluid having an oxygen concentration of at least 95 mole percent.
As used herein the term xe2x80x9cfeed airxe2x80x9d means a mixture comprising primarily oxygen and nitrogen, such as ambient air.
As used herein the terms xe2x80x9cupper portionxe2x80x9d and xe2x80x9clower portionxe2x80x9d mean those sections of a column respectively above and below the mid point of the column.
As used herein the term xe2x80x9cdirect heat exchangexe2x80x9d means the transfer of heat through contact of cooling and heating entities.
As used herein the term xe2x80x9cgas turbinexe2x80x9d means a system comprising a compressor, combustor and expander which produces power.
As used herein the term xe2x80x9cfacilitating columnxe2x80x9d means a system comprising a column and a top condenser which processes a feed comprising oxygen and produces a liquid having an oxygen concentration which exceeds that of the feed.